Warranty is NOT worldwide. It never is.
However, it is somewhat unified in the EU. If bought in a EU country, the N900 is under warranty for 2 years everywhere in the EU. If i remember it right, it's only 1 year in USA and may not be applicable outside it (otherwise americans could just have it repaired in europe in the 2nd year).
As for flashing: you don't sign anything when you buy, so you can put whatever you want (if it works) on your device. It's yours. It may possibly void warranty, but i seriously doubt that. And if the images used are freely available, you can flash whichever you want